I. Field of the Invention
The present invention relates generally to capacitors and, more particularly, to electrochemical capacitors also known as supercapacitors.
II. Description of Related Art
Capacitors have long been known and used in electronic circuitry for the storage of electrical energy. In its simplest form, the capacitor includes a pair of electrically conductive plates, typically constructed of metal, separated by air or a dielectric material. The size or area of the conductive plates as well as the permittivity and thickness of the dielectric material between the plates determines the magnitude of the capacitance of the capacitor.
Some previously-known capacitors include both electrostatic as well as electrolytic capacitors. Such capacitors have a relatively low capacitance, but are able to operate at voltages ranging from a few volts to thousands of volts. Furthermore, such capacitors are capable of operating at high frequencies.
More recently, electrochemical capacitors, also known as supercapacitors, have been developed which do not have a conventional dielectric. Instead, supercapacitor electrodes include a conductive plate, known as a current collector, which is coated with a carbon derivative material, such as activated carbon or graphene. These electrodes are typically separated from each other by an intervening separator made from a porous insulating material that prevents the electrical shorting of the electrodes, but allows electrolyte ions to move between the electrodes. In use, when subjected to a voltage, ions flow between the electrodes results in energy storage within the electrodes through the charge separation at the electrode surface with positive charges in one electrode attracting negative ions to that electrode's surface and with negative charges in the other electrode attracting positive ions to that electrode's surface.
A primary advantage of supercapacitors is that they are able to exhibit capacitances up to 10,000 times that of an electrolytic capacitor. Furthermore, such supercapacitors exhibit the greatest energy density of all currently known capacitors.
FIG. 1 shows a sectional view of a prior art supercapacitor 10. The supercapacitor 10 includes a pair of metal current collectors 12 and 14 which are spaced apart. A porous separator 16 made of an electrical insulating material is positioned in between the two metal current collectors 12 and 14. Electrodes 18, formed of carbonaceous material 20, such as activated carbon, carbon nanotubes, or graphene, are deposited on each current collector 12 and 14. The separator 16 and carbonaceous material 20 are impregnated with an electrolyte (not shown) containing mobile positive and negative ions.
During both charging and discharging of the prior art supercapacitor 10, electrolytic ions must travel between particles or flakes of the carbonaceous material 20 in order to access the full surface area of the electrodes 18 so as to store as much energy as the device can hold. Since the electrolytic ions cannot travel directly through the particles or flakes of carbonaceous material, the ions must travel around them. One exemplary long and tortuous path of an ion traveling through the particles or flakes of carbonaceous material 20 is shown at 24 and effectively increases the ionic impedance of the electrode.
Supercapacitors do, however, suffer from some disadvantages due, in part, to the extended path necessary for the ions to travel during both charge and discharge. First, the previously known supercapacitors were limited to operate at low frequencies, typically less than 1 Hertz. Such low frequencies, of course, limit their applications in electrical circuits.
A still further disadvantage of these previously known supercapacitors is that the rate of charging, as well as discharging, is relatively slow. This relatively slow charging and discharging of the supercapacitor is due primarily to the ionic impedance present in the carbon based electrodes used for the supercapacitor.